A platinum-based intermetallic phase with an early d-metal, Pt 3Ti, has been synthesized in the form of nanoparticles (NPs) dispersed on silica (SiO2) supports. The organometallic Pt and Ti precursors, Pt(1,5-cyclooctadiene)Cl2 and TiCl 4(tetrahydrofuran)2, were mixed with SiO2 and reduced by sodium naphthalide in tetrahydrofuran. Stoichiometric Pt 3Ti NPs with an average particle size of 2.5 nm were formed on SiO2 (particle size: 20-200 nm) with an atomically disordered FCC-type structure (Fm3̄m; a = 0.39 nm). A high dispersivity of Pt 3Ti NPs was achieved by adding excessive amounts of SiO2 relative to the Pt precursor. A 50-fold excess of SiO2 resulted in finely dispersed, SiO2-supported Pt3Ti NPs that contained 0.5 wt % Pt. The SiO2-supported Pt3Ti NPs showed a lower onset temperature of catalysis by 75 °C toward the oxidation reaction of CO than did SiO2-supported pure Pt NPs with the same particle size and Pt fraction, 0.5 wt %. The SiO2-supported Pt3Ti NPs also showed higher CO conversion than SiO2-supported pure Pt NPs even containing a 2-fold higher weight fraction of Pt. The SiO2-supported Pt3Ti NPs retained their stoichiometric composition after catalytic oxidation of CO at elevated temperatures, 325 °C. Pt3Ti NPs show promise as a catalytic center of purification catalysts for automobile exhaust due to their high catalytic activity toward CO oxidation with a low content of precious metals.